The present invention relates to a heater for heating a heating object such as the gas or liquid, and a glow plug for the diesel engine.
The conventional heater will be described below by way of example for a glow plug with reference to FIG. 5. This glow plug 100 is roughly made up of a cylindrical metal shell 101, a heating tube 102 that is cylindrical and has a uniform thickness with a top end cloned, the heating tube 102 fitted inside the metal shell 101, and a center electrode 103 of round pole type. The center electrode 103 is passed through the center of the metal shell 101 to reach the heating tube 102, in which the closed top end of the heating tube 102 and the top end of the center electrode 103 are electrically connected via a heating coil 104. The metal shell 101 has an engagement portion 105 with an aperture (diameter) restricted (narrowed), and the heating tube 102 is fitted into the engagement portion 105 in interference fit. A rear end side of the heating tube 102 is passed through the engagement portion 105 of the metal shell 101 to protrude into the inside of the metal shell 101.
One of the universal technical themes for the glow plug is a reduction in size and weight. For this purpose, it is indispensable that the outer diameter of the heating tube 102 is reduced. At present, the heating tube 102 having a diameter of 3.5 mm has been developed. The technical theme for reducing the size and weight generally applies to all the heaters.
If the heating tube 102 is made slender, the strength is lowered, resulting in a high risk that the heating tube 102 is bent when press fitted into the engagement portion 105 of the metal shell 101. Also, in the case where the heating tube 102 having a uniform thickness is press fitted into the engagement portion 105 of the metal shell 101, it is required that the heating tube 102 starts to be press fitted from the time when the rear end of the heating tube 102 enters the engagement portion 105, and continues to be press fitted even after the rear end of the heating tube 102 gets out of the engagement portion 105. Accordingly, the heating tube 102 is pushed into the inside of the metal shell 101 by a long distance, while being press fitted, whereby the press-fit load is likely to be great. Since there are many sections of the heating tube 102 contributing to (or involving) the press fitting, there is a great dispersion in press-fit load of the heating tube 102, which is caused by variations in frictional force occurring with the engagement portion 105 of the metal shell 101, resulting in poor controllability for the heating tube 102. The magnitude of dispersion in press-fit load has been confirmed through the test of fitting the heating tube having a uniform thickness into the metal shell, as conducted by the present applicant. This dispersion in press-fit load makes it difficult to control the automatic fitting device for the heating tube 102, developing a factor of aggravating the product accuracy.
In recent years, a glow plug is well known in which the heating tube has a two stage structure of a larger diameter portion on the base end side and a smaller diameter portion on the top end side to attain a temperature up performance at the early time, and the larger diameter portion of the heating tube is fitted into the engagement portion of the metal shell in interference fit. However, in the glow plug with this constitution, the press-fit load for the heating tube into the metal shell is likely to increase, because the outer diameter of the larger diameter portion is uniform in thickness. In fitting the heating tube having the two stage structure into the metal shell, in the case where it is designed to allow a part of the smaller diameter portion to be entered to a position of the engagement portion in the metal shell, the following problem arises, even though the heating tube is fitted into the metal shell. That is, since there is a section at the top end portion of the engagement face in the engagement portion not contributing to an interference fit with the heating tube (smaller diameter portion), if the glow plug drops in a process for manufacturing the glow plug or mounting it on the engine to exert a strong impact force toward the base end side of the heating tube from the outside, the heating tube is deviated toward the base end side of the metal shell, or gets rid of the metal shell by any chance.
The present invention has been achieved in the light of the above-mentioned problems, and it is an object of the invention to provide a heater and a glow plug that are highly reliable, in which the press-fit load for the heating tube into the metal shell is suppressed from increasing more than needed, without significant resistance in passage through the engagement portion at the early time of fitting the heating tube into the metal shell.
According to one embodiment of the present invention, there is provided a heater having a cylindrical metal shell formed with an engagement portion having a restricted aperture, and a cylindrical heating tube with a top end side closed to be fitted into the engagement portion in interference fit, in which a base end side of the heating tube is passed through the engagement portion to protrude into the inside of the metal shell, the heating tube comprising a fitting portion that is fitted into the engagement portion formed in the metal shell, a smaller diameter portion formed containing an end portion on the base end side protruding into the inside of the metal shell and having a smaller outer diameter than an inner diameter of the engagement portion, a large diameter portion on the base end side formed between the fitting portion and the smaller diameter portion and having an outer diameter larger than or equal to the inner diameter of the engagement portion, and a larger diameter portion on the top end side formed adjacent to a top end side of the fitting portion and having an outer diameter larger than or equal to the inner diameter of the engagement portion. Also, this invention provides a glow plug of the same structure in which the use of the heater is limited to starting aids of the diesel engine.